1058
Organometallics 1997, 16, 1058-1064
Central versus Terminal Attack in Nucleophilic Addition to (π-Allyl)palladium Complexes. Ligand Effects and Mechanism Attila Aranyos, Ka´lma´n J. Szabo´, Ana M. Castan˜o, and Jan-E Ba¨ckvall* Department of Organic Chemistry, University of Uppsala, Box 531, S-751 21 Uppsala, Sweden Received November 8, 1996X
Nucleophilic addition to (η3-2-chloropropenyl)palladium complexes 1 with stabilized carbanions such as dialkyl malonates was studied. These complexes are used as probes to determine whether nucleophilic attack occurs at the central or terminal carbon of the π-allyl group. Attack at the central carbon leads to substitution of chloride via a palladacyclobutane intermediate. The regiochemistry of the reaction (central versus terminal attack) is controlled by proper choice of ligands. Thus, σ-donor ligands direct the attack of the nucleophile to the central carbon (C-2) of the allyl group whereas π-acceptor ligands direct the attack to the terminal carbons (C-1 or C-3). It was found that there is a correlation between the relative rate of central versus terminal attack and the 13C NMR shifts of the allyl group. The shift difference between the central and terminal carbons, Cc - Ct, can be used to predict the site of attack. Ab initio calculations were performed on (π-allyl)palladium complexes as well as on the postulated palladacyclobutane. The calculations support the experimental results, and for the π-allyl complexes with the σ-donor ligands the LUMO from the calculations is the symmetrical orbital with a large coefficient at the central carbon. Introduction (π-Allyl)palladium complexes are important intermediates in a number of catalytic reactions such as allylic substitution,1 allylic oxidation,2 and 1,4-oxidation of conjugated dienes.3 These reactions involve nucleophilic addition of heteroatom or carbon nucleophiles on the allyl moiety. The ancillary ligands involved, as well as the nature of the nucleophile, play an important role in both the rate and the regiochemical outcome of these reactions.1,4-6 In general the reaction proceeds faster in the presence of π-acceptor ligands and the nucleophile Abstract published in Advance ACS Abstracts, February 1, 1997. (1) (a) Godlesky, S. A. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, U.K., 1991; Vol. 4, p 585. (b) Trost, B. M.; Verhoven, T. R. J. Am. Chem. Soc. 1980, 102, 4730. (c) Trost, B. M.; Verhoven, T. R. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. A., Eds.; Pergamon: Oxford, U.K., 1982; Vol. 8, p 799. (d) Tsuji, J. Acc. Chem. Res. 1969, 2, 144. (2) (a) Hansson, S.; Heumann, A.; Rein, T.; Åkermark, B. J. Org. Chem. 1990, 55, 975. (b) Ba¨ckvall, J. E.; Hopkins, B. R.; Grennberg, H.; Mader, M. M.; Awasthi, A. K. J. Am. Chem. Soc. 1990, 112, 5160. (3) (a) Castan˜o, A. M.; Ba¨ckvall, J. E. J. Am. Chem. Soc. 1995, 117, 560. (b) Ba¨ckvall, J. E. Pure Appl. Chem. 1992, 64, 429. (c) Ba¨ckvall, J. E.; Bystro¨m, S.; Nordberg, R. E. J. Org. Chem. 1984, 49, 4619. (d) Ba¨ckvall, J. E.;. Nystro¨m, J. E.; Nordberg, R. E. J. Am. Chem. Soc. 1985, 107, 3676. (4) (a) Szabo´, K. J. Organometallics 1996, 15, 1128. (b) Formica, M.; Musco, A.; Pontellini, R.; Linn, K.; Mealli, C. J. Organomet. Chem. 1993, 448, C6-C9. (c) Åkermark, B.; Zetterberg, K.; Hansson, S.; Krakenberger, B.; Vitagliano, A. J. Organomet. Chem. 1987, 335, 133. (5) Hegedus, L. S.; Darlington, W. H.; Russell, C. E. J. Org. Chem. 1980, 45, 5193. (6) (a) Hoffmann, H. M. R.; Otte, A. R.; Wilde, A.; Menzer, S.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 100. (b) Castan˜o, A. M.; Aranyos, A.; Szabo´, K. J.; Ba¨ckvall, J. E. Angew. Chem., Int. Ed. Engl. 1995, 34, 2551. (c) Carfagna, C.; Mariani, L.; Musco, A.; Sallese, G.; Santi, R. J. Org. Chem. 1991, 56, 3924. (d) Carfagna, C.; Galarini, R.; Musco, A. J. Mol. Catal. 1992, 72, 19. (e) Wilde, A.; Otte, A. R.; Hoffmann, H. M. R. J. Chem. Soc., Chem. Commun. 1993, 615. (f) Hoffmann, H. M. R.; Otte, A. R.; Wilde, A. Angew. Chem., Int. Ed. Engl. 1992, 31, 234. (g) Otte, A. R.; Wilde, A.; Hoffmann, H. M. R. Angew. Chem., Int. Ed. Engl. 1994, 33, 234. (h) Carfagna, C.; Galarini, R.; Linn, K.; Lo´pez, J. A.; Mealli, C.; Musco, A. Organometallics 1993, 12, 3019. X
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attacks either terminus (C-1 or C-3) of the allyl moiety. However, with some σ-donor ligands and with less stabilized carbon nucleophiles (pKa 20-30) nucleophilic attack at the central carbon to yield cyclopropane derivatives has been demonstrated (eq 1).5,6
Hegedus5 reported the first example of this type of reaction and proposed the mechanism outlined in eq 1, and since then, several other examples have been reported.6 Hoffmann6a has proven this mechanism by isolating and characterizing the proposed palladacyclobutane7,8 intermediate. In a preliminary communication6b we reported that also more stabilized carbon nucleophiles such as branched dialkyl malonates attack the central carbon of (π-allyl)palladium complexes under certain conditions. We now give a full account of the previous study, report additional results on ligand effects, correlate the relative reactivity of central versus terminal attack with 13C NMR shifts, (7) Recent review on metallacyclobutane complexes of the group eight transition metals: Jennings, P. W.; Johnson, L. L. Chem. Rev. 1994, 94, 2241. (8) For related metallacyclobutane intermediates from central attack on (π-allyl)metal complexes, see: (a) Ohe, K.; Matsuda, H.; Morimoto, T.; Ogoshi, S.; Chatani, N.; Murai, S. J. Am. Chem. Soc. 1994, 116, 4125. (b) Tjaden, E. B.; Casty, G. L.; Stryker, J. M. J. Am. Chem. Soc. 1993, 115, 9814. (c) Wakefield, J. B.; Stryker, J. M. J. Am. Chem. Soc. 1991, 113, 7057. Tjaden, E. B.; Stryker, J. M. Organometallics 1992, 11, 16. (e) Ephritikhine, M.; Green, M. L. H.; MacKenzie, R. E. J. Chem. Soc., Chem. Commun. 1976, 619. (f) Ephritikhine, M.; Francis, B. R.; Green, M. L. H.; MacKenzie, R. E.; Smith, M. J. J. Chem. Soc., Dalton Trans. 1977, 1131. (g) Adams, G. J. A.; Davies, S. G.; Ford, K. A.; Ephritikhine, M.; Todd, P. F.; Green, M. L. H. J. Mol. Catal. 1980, 8, 15.
© 1997 American Chemical Society
(π-Allyl)palladium Complexes Scheme 1
Organometallics, Vol. 16, No. 5, 1997 1059 Table 1. Variation of Ligands and Nucleophilesa entry complex
provide support from theoretical calculations, and discuss the different factors governing whether central or terminal attack occurs. Results and Discussion A. Preparation of (π-Allyl)palladium Complexes. The (η3-2-chloropropenyl)palladium complex 1a was prepared from 2,3-dichloropropene and palladium chloride in the presence of carbon monoxide (eq 2).9a
Complex 1a is a known compound9b for 30 years, and such 2-chloro π-allyl complexes of platiunum and palladium were recently generated in situ in catalytic reactions for studying the site of nucleophilic attack.8a The cationic complexes 1b-q were prepared in situ from the chloro dimer 1a either by treatment with AgBF4 and subsequent addition of the appropriate ligands or just by adding the ligands to the solution of 1a (eq 3). The cationic tetrafluoroborate complexes were characterized by their 1H and 13C NMR spectra. B. Reaction of (π-Allyl)palladium Complexes 1 with Carbon Nucleophiles. The (π-allyl)palladium complexes 1 were reacted with sodium diethyl methylmalonate, sodium dimethyl methylmalonate, or sodium methyl methylacetoacetate, in the presence of different ligands (Scheme 1). Attack by the nucleophile at the terminal carbon leads to the monoalkylated product 4, which can be isolated and characterized. On the other hand, an initial attack at the central carbon, followed by the elimination of the chloride and subsequent addition to the terminal carbon, yields the doubly alkylated product 3. Cyclopropane formation from the 2-chloropalladacyclobutane intermediate is unlikely and has not been observed under the reaction conditions employed in this study. (9) (a) Auburn, P. R.; MacKenzie, P. B.; Bosnich, B. J. Am. Chem. Soc. 1985, 107, 2033. Dent, W.; Long, R.; Wilkinson, A. J. J. Chem. Soc. 1964, 1585. (b) Lupin, M. S.; Powell, J.; Shaw, B. L. J. Chem. Soc. A 1966, 1687.
ligand (equiv)
nucleophile
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1b 1c 1d 1e 1f 1g 1h 1j 1k 1l 1m 1n 1o 1p 1q 1b 1d 1j
TMEDA (2) NaC(Me)(CO2Et)2 bipy (2) TMPDA TMMDAc TMBDAc dppb (2) P(OPh)3 (6) PPh3 (4) dppf (2) PBu3 (4) dppe (2) COD DMSO BuS(CH2)2SBu phenyl-BIP10 TMEDA NaC(Me)(CO2Me)2 TMPDA PPh3
19
1b
TMEDA
Na(Me)C
products 3:4b >99:99:99:99:
